Site-Specific Assessment of Jack-Ups Part 5: When Things Go Wrong Lessons and Mitigations

Size: px

Start display at page:

Download "Site-Specific Assessment of Jack-Ups Part 5: When Things Go Wrong Lessons and Mitigations"

Cordelia Hall
5 years ago
Views:

1 OIL & GAS Site-Specific Assessment of Jack-Ups Part 5: When Things Go Wrong Lessons and Mitigations Noble Denton marine services Mike White 1 SAFER, SMARTER, GREENER

2 Overview Review of several incidents: Leman Delta. High Island 2. Punch-through recovery. Some Conclusions. 2

3 "Monarch" at Leman Delta Leman Delta (1974): 34m WD, not normally manned well-head platform. "Monarch" (1986), F&G L780 Mod V, 350 WD, 3350t VDL 3

4 "Monarch" at Leman Delta "Monarch" leg well and jacking system: 34m WD Not normally manned well-head platform. 4

5 "Monarch" at Leman Delta "Monarch" leg-hull load transfer on rack-chocks (left) and when jacking (right) Bending Moment Diagram Shear Force Diagram Bending Moment Diagram Shear Force 5

6 "Monarch" at Leman Delta Leman Delta seabed: Large sand-wave. Scour prevalent. Monarch has relatively flat spudcan. 6

7 "Monarch" at Leman Delta Installed condition: 32m WD 25m WD 7

8 "Monarch" at Leman Delta Rack Phase Differential (RPD) Due to shearing of individual faces of the leg. C The leg is constrained within the guides so the shear can be measured by relative vertical movement at the three chords. B B2 A2 A RPD (face AB) = A - B RPD (face AC) = A - C RPD (face BC) = B - C B1 Chord C Chord B A1 Chord A 8

9 "Monarch" at Leman Delta Top of port jack-frame & measuring RPD. 9

10 "Monarch" at Leman Delta 10

11 "Monarch" at Leman Delta 11

12 "Monarch" at Leman Delta 12

13 "Monarch" at Leman Delta 13

14 "Monarch" at Leman Delta 14

15 "Monarch" at Leman Delta 15

16 "Monarch" at Leman Delta RPD is a good measure of the shear in the leg. Leg distorts into 3 straight parts, above, between and below the guides. For compatibility the chords undergo large local bending at the transitions. Guides carry large reactions & will wear during jacking. Guide forces compress/stretch braces - changing chord spacing. Lower guide Upper guide 16

17 "Monarch" at Leman Delta Recovery of the Unit: PROBLEMS: Eccentric spudcan loads. Buckled braces in leg between guides. Bent legs with sway mode. Shell Platform Leaning towards Shell platform. Chords moving out of alignment with guides. Hull very close to Shell platform. Rack-chocks difficult to disengage. 17

18 "Monarch" at Leman Delta Recovery of the jack-up: Large eccentric spudcan support generates severe loading conditions when running on pinions. Reasonably easy to straighten the legs when stationary by differential jacking using local panel. However this generates large loads on pinions and once pinions are running the RPD will return. Tilting the jack-up helps reduce eccentric loading. Repeated short jacking stages help limit the development of shear and consequent damage. Monitor all stages and proceed with caution. 18

19 "Monarch" at Leman Delta Questions for wet tow. Strength of damaged leg? Allowable sea state? Additional buckling / tearing? Falling objects? 19

Hurricane Rita regional imagery, 2005.09.

20 High Island 2 High Island 2 was close to the track of hurricane Rita. Hurricane Rita regional imagery, at 1715Z. Center-point Latitude: 27:50:46N Longitude: 92:10:39W 20

"High Island 2" 092412 092418 092500 85 55 35 MP1 092406 105 HI1 GSF Rigs 092400 110 MP4 HI2 "High Island HI8 2" (85 knot 1-min wind speed) DDII A3 HI4 HI3 A7 35 25 CS 092318 110 092312 120 120

21 "High Island 2" MP HI1 GSF Rigs MP4 HI2 "High Island HI8 2" (85 knot 1-min wind speed) DDII A3 HI4 HI3 A CS Rita 1-min wind speed (kts)and time (GMT)indicated along track

22 "High Island 2" Adriatic 7 after Rita High Island 3 after Rita 22

23 "High Island 2" Adriatic 7 High Island 3 23

24 "High Island 2" Bow leg penetrated additional 6 ft. Resulted in forward tilt of 3⁰. Movement away from platform. Noble Denton post-analysis identified the additional penetration of the bow leg. 24

25 "High Island 2" Hull and leg angles meant that spudcan vertical reactions generate leg moments at the hull level. The leg moments are initially carried largely by the pinions GSF High Island 2 Leg and Jack-Frame Elevations When jacking, the pinion loads equalize, releasing the moment they carried initially. Hence all leg moments at the hull must be carried by the leg braces. The braces were not strong enough for the post-hurricane large angle loading condition. Height above BL (ft) Face Width Leg (ft) 25

26 "High Island 2" Initial levelling caused leg RPD and K-joint failures. this is the distortion indicated by analysis. Upper Guide Lower Guide Bending Moment 26

27 "High Island 2" And visible kinks in the leg chords. 27

28 "High Island 2" Jacking on one leg: Initial condition is that hull is sloping forward levelling the hull will reduce the moments on the legs. CG Total shear force perpendicular to all the legs is given by: Weight x sin( ) i.e., 9967 kips x sin(3.3º) = 569 kips. Reducing this leg shear by levelling the hull will reduce the resulting leg moments at the hull. 28

29 "High Island 2" Jacking on one leg down: Hull can be levelled by jacking the bow leg down through the hull (jacking the hull up at the bow) the hull and legs then rotate about an axis through the other (aft) spudcans. The total leg shear forces and moments reduce as the inclination reduces. CG Jack this leg down through the guides 29

30 "High Island 2" Calculations showed that the soils were providing a significant degree of support to the system in the slumped-forward condition. CG The soil moments helped to resist the effects of the leg shears caused by the angle of the hull. Levelling the hull would rotate the spudcans in the soil. The spudcan moment would then reverse and resist the jacking attempt. Calculations indicated that this would overload the pinions and lead to collapse. 30

31 "High Island 2" WHAT TO DO???? 1) Use explosives to sever legs and then remove pieces but 50 ft airgap. 31

32 "High Island 2" WHAT TO DO???? 2) Or use a crane vessel to lift the hull. Large spreader beams required and major connection problems. Limited availability of suitable crane vessels. 32

33 "High Island 2" WHAT TO DO???? 3) Dredge around legs : To make rotation of spudcans easier equipment was put on standby. but this loosens soil supporting the jack-up. Also possibility of spudcans sliding into holes and splaying legs. 33

34 "High Island 2" WHAT TO DO???? 4) Hand of God = External force An external pull-back force reverses the shears in the leg and reduces the moments at the hull level. pull-back Force CG The pull-back force must be large enough to overcome the hull sway force and the spudcan resistance effect. 34

35 "High Island 2" 4) External force produces changes in leg moments. 0 kip pull-back Bending about port-stbd axis 1200 kip pull-back Bending about port-stbd axis 35

36 "High Island 2" 4) External force produces changes in leg shears. 0 kip pull-back Shear along fwd-aft axis 1200 kip pull-back Shear along fwd-aft axis 36

37 "High Island 2" Moment in the legs reduces as the hull is jacked level also, the moments are reduced if there is a pull-back force. Aft Leg Moment 200, , ,000 0 Kip Pull Back 600 Kip Pull Back 1200 kip Pull Back Moment (kip.ft) 50, , , , ,000 Lift (Bow leg) ft 37

"High Island 2" Leg moments at lower-guide: Needed to reduce bow leg moment so the RPD would reduce and the leg would feed through the guides. Lower Guide Moment (Newton.Metres) 1.2E+08 1.0E+08 8.

38 "High Island 2" Leg moments at lower-guide: Needed to reduce bow leg moment so the RPD would reduce and the leg would feed through the guides. Lower Guide Moment (Newton.Metres) 1.2E E E E E E E+07 Y moment variation with pull back force 0.0E+00 0% -2.0E+07 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 110% 120% -6.0E E+07 % of 1200 kips pull back Elastic soils, Leg 1 Elastic soils, Leg 2 Elastic soils, Leg 3 Plastic soils, Leg 1 Plastic soils, Leg 2 Plastic soils, Leg 3 38

39 "High Island 2" The Plan. pull-back barge 39

40 "High Island 2" Jacking the hull up the bow leg moves High Island 2 backwards. Change in Pull Back Force with Bow Leg Jacking Distance This automatically reduces the pull-back tension as the pull-back hawser is relaxed. 6 ft jacking reduces tension to about 1/3 of original. Tension in Pull Back Lines (kip) 1,400 1,200 1, Pull Back Force (Kip) Bow Leg Jacking Distance (ft) 40

41 "High Island 2" Jacking the Hull up the Bow Leg moves the HI-2 backwards This automatically reduces the pull-back tension as the Dyneema is relaxed 6.ft jacking reduces tension to about 1/3 rd original 41

42 "High Island 2" SUCCESS! The key to understanding jack-up behavior is knowing the system moment and how it is being carried. When facing challenges outside the normal range, take your time and seek expert advice. 42

43 Punch-Through Recovery Often: Operations Manuals do not cover this eventuality. If they do, the guidance may be OK for small out of level, but not for a serious punchthrough. There are elements of good practice that apply to most types of jack-up. 43

44 Punch-Through Recovery Much of the previous discussion is applicable the key elements to minimizing the consequences are: Before the event: Minimize the loads involved by preloading one leg at a time. Preload with the hull in, or very close to, the water. These options are not all plain-sailing: Preloading takes longer. Challenging to plan & execute in areas with large tidal ranges and / or short weather windows. Understand your jack-up and its capability the deeper the water the greater the significance of a punch-through event. 44

45 Punch-Through Recovery After the event: Call for engineering support. Reduce the challenge dump / pump-out all possible ballast & dump / backload variable load where at all possible. Remember the system moment. If jacking-down to lower the high side on two legs, the moment will end up on the non-jacked punch-through leg. This is a bad plan (although maybe do-able by an operator experienced in singlechord jacking). In any event, jack only one leg at a time. Instead, aim to jack-up the punch-through leg. Maximize the external help: Use the tide to advantage. Consider using tugs to assist. Jack in small increments (monitor legs, guides, RPD, etc.). Reference: Noble Denton Guideline Document 0009/ND, Appendix D. 45

46 Punch-Through Recovery The problem can be analyzed. Vertical foundation load during preloading (tonnes) Punchthrough at Full Preload Preload 10 5 Average Upper Bound 20 Spudcan tip penetration (m) Spudcan stillwater reaction Spudcan preload reaction Stillwater Start Punchthrough = 3.6m End Puncthrough = 3.65m Punchthrough = 0.05m Spudcan tip penetration (ft) Vertical foundation load during preloading (kips) 46

47 Punch-Through Recovery The outcomes can be investigated. Buoyancy (tonnes) Buoyancy Pick up 10,000 9,000 8,000 7,000 6,000 5,000 4,000 3,000 2,000 1, Hull Trim (degrees) 0.75m Draught Moment Bow DownPick up 0.75m Draught Port Down 250, ,000 Righting Moment (tonnes.m) 150, ,000 50, Hull Trim (degrees) 0.75m Draught Bow Down 0.75m Draught Port Down 47

48 Punch-Through Recovery In this case the buoyancy pick-up is rapid and the punch-through should be very limited and controllable. 48

49 Conclusions We have reviewed three scenarios: In each case, the key to understanding the jack-up s behavior is knowing the system moment and how it is being carried. There is generally a way forward but it may take careful thinking to get there. Improve your chances back-load everything you can 10% reduction in total hull weight buys about 10% improvement in results and could make the difference between failure and success. When facing challenges outside the normal range, be sure to know what to expect when you push the buttons - if you don t things get broken. It may help to seek advice from an expert. Punch-through survivability (or otherwise) can be analyzed. 49

50 Questions? SAFER, SMARTER, GREENER 50

Site-Specific Assessment of Jack-Ups Part 1: Jack-Up Basics & Why We Do Site-Specific Assessments

OIL & GAS Site-Specific Assessment of Jack-Ups Part 1: Jack-Up Basics & Why We Do Site-Specific Assessments Noble Denton marine services Mike White 1 SAFER, SMARTER, GREENER Overview What are we talking